1. Field of the Invention
The present invention relates to a current-voltage conversion circuit, a current compression and extension circuit, an automatic exposure control system, and an automatic exposure control system with a built-in sensor, all utilizing CMOS processes.
2. Description of the Background Art
(a-1) First Background Art
FIG. 11 is a circuitry diagram of a conventional current-voltage conversion circuit 201. The current-voltage conversion circuit 201 comprises an operational amplifier 53 and a diode 54. A current input terminal 51 is connected to a reverse input terminal of the operational amplifier 53 while a first reference voltage input terminal 52 is connected to a non-reverse input terminal of the operational amplifier 53. A voltage output terminal 55 and a cathode of the diode 54 are connected commonly to an output terminal of the operational amplifier 53. An anode of the diode 54 is connected to the current input terminal 51.
When a reference voltage V.sub.REF1 is applied to the first reference voltage input terminal 52 and a current I is supplied to the current input terminal 51, the current I flows into the diode 54 so that a voltage V.sub.BE which is obtained by compressing the current I by logarithmic compression is developed across the diode 54. ##EQU1## where q: electric charge of an electron
k: Boltzmann's constant PA1 T: absolute temperature PA1 I.sub.s : reverse saturation current
Hence, at the voltage output terminal 55, a voltage V.sub.OUT is outputted which is obtained by subtracting the logarithmically compressed voltage which is developed across the diode 54 from the reference voltage V.sub.REF1 (See Eq. 2 below). ##EQU2##
(a-2) Second Background Art
FIG. 12 is a circuitry diagram of a conventional current compression and extension circuit 202. The current compression and extension circuit 202 is the same as the current-voltage conversion circuit 201 of FIG. 11 as it is modified to further comprise an NPN transistor 57. That is, the NPN transistor 57 has its emitter connected to the output terminal of the operational amplifier 53, its base connected to a second reference voltage input terminal 58 and its collector connected to an output terminal 56. Since a voltage which is expressed by Eq. 2 is supplied to the emitter of the transistor 57, when a reference voltage V.sub.REF2 is applied to the second reference voltage input terminal 58, a current which is expressed by Eq. 3 is obtained from the output terminal 56. ##EQU3##
(a-3) Third Background Art
FIG. 13 is a circuitry diagram of a conventional automatic exposure control system 203. The automatic exposure control system 203 detects a current by an optical sensor and converts the current into a voltage in order to perform its intended control.
The automatic exposure control system 203 comprises operational amplifiers 62, 67 and 68, diodes 63 and 66 and current sources 61 and 65. To supply a current which is the same in volume as a current which is available from the optical sensor, the current source 61 is inserted between a reverse input terminal and a non-reverse input terminal of the operational amplifier 62. A voltage source 64 supplies a reference voltage V.sub.REF1 to the non-reverse input terminal of the operational amplifier 62. An anode of the diode 63 is connected to the reverse input terminal of the operational amplifier 62. A cathode of the diode 63 is connected to an output terminal of the operational amplifier 62 which is connected to a non-reverse input terminal of the operational amplifier 67.
A reverse input terminal of the operational amplifier 67 is connected to the current source 65 and a cathode of the diode 66. An anode of the diode 66 is connected to the output terminal of the operational amplifier 67.
The output terminal of the operational amplifier 67 is further connected to a non-reverse input terminal of the operational amplifier 68. A reverse input terminal and an output terminal of the operational amplifier 68 are connected commonly to an output terminal 69.
When the current source 61 supplies a current I, a voltage available at the output terminal of the operational amplifier 62 is expressed by Eq. 2. Hence, by ensuring that the current source 65 supplies a current I.sub.O and setting reverse saturation currents I.sub.S of the diodes 63 and 66 equal to each other, a voltage V.sub.67 expressed by Eq. 4 is obtained at the output terminal of the operational amplifier 67. ##EQU4##
The operational amplifier 68 forms a voltage follower circuit to the voltage V.sub.67 which appears at the output terminal of the operational amplifier 67, and therefore supplies the voltage V.sub.67 to the output terminal 69. This allows the automatic exposure control system 203 to detect the current I. Based on the detected current I, the automatic exposure control system 203 outputs the voltage V.sub.67 which is controlled by the reference voltage V.sub.REF1 and the current I.sub.O while suppressing an impedance at the output terminal 69 low.
(a-4) Fourth Background Art
FIG. 14 is a circuitry diagram of a conventional automatic exposure control system with a built-in sensor 204. The automatic exposure control system with a built-in sensor 204 is the same as the automatic exposure control system 203 as it is modified to replace the current source 61 with an optical sensor 70. In the system 204, Eq. 4 is satisfied if a current flowing in the optical sensor 70 has a value I.
In the conventional current-voltage conversion circuit, the conventional current compression and extension circuit, the conventional automatic exposure control system, and the conventional automatic exposure control system with a built-in sensor above, the operational amplifiers 53, 62, 67 and 68 are formed using CMOS processes. On the other hand, the diodes 54, 63 and 66 are formed by bipolar transistors. Bipolar processes are also used to form the transistor 57 of the current compression and extension circuit 202.
To build these circuits and systems, not only CMOS processes but also bipolar processes are necessary. In other words, CMOS processes alone are insufficient to realize these circuits and systems. Bipolar processes are also needed.